Semiconductor laser diodes have numerous advantages. They are small and the widths of their active regions are typically a submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor laser diodes produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (˜800 nm) which is near the absorption spectrum of common solid state laser rods and slabs made from Neodymium-doped, Yttrum-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode are decreased as the operating temperature in the junction increases.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a specific output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes that are used to pump an Nd:YAG rod or slab should emit radiation at about 808 nm since this is the wavelength at which optimum energy absorption exists in the Nd:YAG. However, for every 3.5° C. to 4.0° C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Accordingly, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by laser diodes, dissipation of the heat becomes more problematic since it becomes necessary to densely pack a plurality of individual diodes into arrays which generate the required amounts of input power for the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes decreases. This aggravates the problem of heat extraction from the arrays of individual diodes.
Laser diode systems must therefore utilize an effective heat transfer mechanism to operate as efficiently as possible. One of the current laser diode systems utilizes a pin fin heat exchanger though which cooling water flows and absorbs the heat. Specifically, the laser diode system has a laser diode bar soldered between two metallic end-blocks. The end-bocks are themselves soldered onto a partially metallized substrate. This package is known as an array submodule. The function of this package is to extract heat from the laser diode bar and allow the connection of electrical hook-ups. Before use, the package is soldered onto a water-cooled heat exchanger. The package generally pulls heat away from both sides of the laser diode bar via the end blocks, and the heat travels down to the pin fin heat exchanger where the heat is carried away by coolant water.
However, a disadvantage of this arrangement is the distance between the heat source of the laser and the water coolant. This distance can cause the package to run at elevated temperatures, e.g., when the laser diode bar is operated above 20 Watts. It also contributes to poor performance when operated in an ON/OFF cycled mode.
Another type of cooling system for a laser diode package utilizes macrochannel coolers. These laser diode packages are small, e.g., 1 mm thick, and have small water channels running though them. The water channels pass close to a bottom side of the heat source (i.e., the laser diode bar), allowing for efficient thermal transfer. However, the macrochannel coolers typically remove heat from only one side of the laser diode bar.
When the macrochannel coolers are used, electrical current and water coolant reside in the same physical space. Consequently, the coolant water must be deionized. However, the use of deionized water requires all parts that are exposed to the water supply be either glass, plastic, stainless steel, or gold-plated. Parts which are not made of these materials usually deteriorate quickly and can cause severe corrosion problems.
Macrochannel coolers are made from a stack of thin copper sheets diffusion-bonded together in multiple layers. Each layer is photoetched so that, after diffusion bonding with other layers, small channels are formed allowing coolant passage through an area underneath the laser diode. However, the macrochannel coolers are relatively large and expensive to make, due to the limitation on the materials of which they are formed. The present invention is directed to satisfying this and other needs.